904
Published on the web September 5, 2011
Bis(cyclopentadienyldicarbonyliron) as a Convenient Carbon Monoxide Source
in Palladium-catalyzed Carbonylative Coupling of Aryl Iodides
with Amines, Alcohols, and Thiols
Ryotaro Nakaya,1,2 Hideki Yorimitsu,*2 and Koichiro Oshima*1,3
1Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510
2Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502
3Environment, Safety, and Health Organization, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501
(Received March 2, 2011; CL-110180; E-mail: yori@kuchem.kyoto-u.ac.jp, oshima@orgrxn.mbox.media.kyoto-u.ac.jp)
Table 1. Optimization of conditions
Bis(cyclopentadienyldicarbonyliron) ([CpFe(CO)2]2) serves
as a carbon monoxide source in carbonylative coupling
reactions. Treatment of aryl iodides with primary amines in
the presence of DBU and [CpFe(CO)2]2 under palladium
catalysis provides the corresponding benzamides in good yields.
Similar reactions with phenols and thiols provide the corre-
sponding benzoate esters and thioesters, respectively. A catalytic
amount of DMAP as an additive promoted the carbon-
ylative coupling reactions with primary alcohol and secondary
amine.
Entry [CpFe(CO)2]2/equiv Ligand
Base Yielda/%
1
2
3
4
5
6
7
1.5
none
none
none
PPh3
DBU
DBU
DBU
DBU
100
96
70
69
79
43
69
0.75
0.38
0.38
0.38
0.38
0.38
Transition-metal-catalyzed carbonylative cross-coupling re-
actions in the presence of carbon monoxide are useful methods
for synthesizing carbonyl compounds.1-3 Because of the inherent
difficulty in handling highly toxic carbon monoxide, consid-
erable efforts have been made to develop carbon monoxide
equivalents to achieve CO-gas-free carbonylation.4 Organic
carbonyl compounds such as pentafluorobenzaldehyde and N,N-
dimethylformamide are known to serve as carbon monoxide
equivalents.5-7 However, the scope of substrates and hence the
diversity of products are limited. Alternatively, metal carbonyl
complexes are also useful as carbon monoxide sources since
they emit carbon monoxide upon heating. [Ni(CO)4] was first
used stoichiometrically in carbonylative reactions.8 Because of
the high toxicity of the nickel complex, palladium-catalyzed
reactions with safer solid metal carbonyls such as [Mo(CO)6],9
[Cr(CO)6],9b and [W(CO)6]9b,9h,9i have been developed.10
Iron is a ubiquitous, inexpensive, and nontoxic transition
metal, and iron compounds have thus been attracting increasing
attention in organic synthesis.11 Despite the superiority of iron
over molybdenum, chromium, and tungsten, [Fe(CO)5] and
[Fe3(CO)12] were reported to be unreactive for palladium-
catalyzed carbonylative coupling.9b This was indeed the case,
and our attempts to use the simple iron carbonyl complexes
resulted in failure. Instead, we found that economical and easy-
to-handle bis(cyclopentadienyldicarbonyliron) ([CpFe(CO)2]2)
serves as a source of carbon monoxide in palladium-catalyzed
carbonylative coupling for synthesizing aromatic amide
(Table 1). Treatment of 4-iodotoluene with aniline in the
presence of a catalytic amount of palladium acetate, [CpFe-
(CO)2]2 (1.5 equiv), and 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) in refluxing toluene for 1 h provided expected amide
1a quantitatively (Entry 1). It is worth noting that other iron
carbonyl complexes such as [CpFe(CO)2I], [Cp*Fe(CO)2]2,
[Cp*Fe(CO)2I], and [Fe2(CO)9] were useless under the reaction
conditions and that almost no conversions were observed.12
P(c-C6H11)3 DBU
DPPE DBU
P(c-C6H11)3 DBN
aThe remainder of mass balance was unreacted 4-iodotoluene.
Since 1.5 equiv of [CpFe(CO)2]2 corresponds to 6 equiv
of carbon monoxide, we thus tried to reduce the amount of
[CpFe(CO)2]2. The amount of [CpFe(CO)2]2 could be reduced
to 0.75 equiv without significantly deteriorating the yield
(Entry 2). In the presence of 0.38 equiv of [CpFe(CO)2]2 (1.5
equiv of CO), the reaction proceeded cleanly to afford a 70%
yield of 1a, the remainder of mass balance being unreacted
4-iodotoluene (Entry 3). To increase the efficiency of the
reaction with 0.38 equiv of [CpFe(CO)2]2, we further examined
the effect of ligand. Tricyclohexylphosphane slightly improved
the yield to 79% (Entry 5) whereas triphenylphosphane and
bidentate 1,2-bis(diphenylphosphano)ethane had no and adverse
effects, respectively (Entries 4 and 6). The choice of strong
organic bases such as DBU and 1,5-diazabicyclo[4.3.0]non-
5-ene (DBN, Entry 7) is critical. Other organic bases such as
1,4-diazabicyclo[2.2.2]octane, tributylamine, and pyridine as
well as inorganic bases such as cesium carbonate and potassium
t-butoxide failed to promote the reaction. In analogy with the
molybdenum-mediated carbonylation,9b DBU would strongly
coordinate to iron, which leads to irreversible release of carbon
monoxide from iron carbonyl complexes.
Finally, the use of 2 equiv of iodobenzene and a prolonged
reaction time could improve the yield of 1b (based on
4-toluidine) while reducing the amount of the palladium catalyst
to 1 mol % (Table 2, Entry 1).13 A variety of benzamides were
synthesized under the optimized reaction conditions. Primary
amines, both aromatic and aliphatic, reacted smoothly to afford
the corresponding benzamides in good yields (Entries 1-7). The
Chem. Lett. 2011, 40, 904-906
© 2011 The Chemical Society of Japan